I need to know if i can safely use normal string or I have to use mb_* string when I work on $_SERVER vars. (UTF-8)
For example $_SERVER['HTTP_HOST'] can contain multibyte chars?
What happens if i make an http request with Host: header containing multybyte chars?
And where a client can specify which econding is using when he makes an http request?
Yes, you should use multibyte. If not for anything else, there are cyrillic domain names, so $_SERVER should naturally include utf8 data.